The invention relates to a digital control device for a fuel injection system of an internal combustion engine, and particularly, to a device for controlling at least one of solenoids of the fuel injection valves in accordance with the rate of air flow which is introduced into the engine and with the signal which is proportional to the number of revolutions of the engine.
Recently, much attention has been focused upon the construction of a control device which can accurately control the air-fuel ratio of an internal combustion engine in order to solve some problems caused by recent strict exhaust emission controls, for instance, controls for promoting fuel economy. Accordingly, many such precise control devices are now available. These devices generally comprise a digital microcomputer used for controlling the amount of fuel injected into the engine so that the controlled amount of fuel corresponds to the amount of air which is introduced into the engine. In a conventional fuel injection system, the injection valves are operated intermittently and are opened for a specific injection period at each stroke of the corresponding cylinder of the engine. Accordingly, the amount q of fuel which is provided to the engine during a specific period of time is expressed as: EQU q=N.multidot.T.multidot.k (1)
where N is the number of revolutions (rpm) of the engine, T is the injection period of the injection valves, and k is a unit amount of fuel injection. The unit amount k depends upon the injection pressure of the injection valve and upon the cross-sectional area of the opening of the injection valve. The air-fuel ratio (A/F) of the internal combustion engine is then defined as: EQU A/F=Q/q=Q/(N.multidot.T.multidot.k) (2)
where Q is the amount of air provided to the engine during a specific period of time. In the above equation (2), the amount of air (Q) introduced into the engine and the number of revolutions of the engine (N) can be respectively measured by means of measuring devices attached to the engine. Therefore, the optimum combustion of the air/fuel mixture can be obtained by controlling the injection period T, which is calculated by the digital computer in accordance with operating parameters of the engine such as the amount of air introduced into the engine or the number of revolutions of the engine, so that the injection period T is proportional to Q/N. However, the control devices employing a digital computer according to the prior art require analogue-digital converters for converting input signals which represent the respective operating parameters of the engine. These analogue-digital converters, particularly those converters for converting an input signal which represents the amount of air introduced into the engine, are extremely expensive to construct. Therefore, it is generally very difficult to reduce the total costs of the conventional control devices employing analogue-digital converters.
Further, in the prior control devices because the calculation of Q/N is performed by a digital computer, a time delay corresponding to the operation time of the computer always occurs in the controlling of the engine.
Another disadvantage of the prior control devices is that the calculation of Q/N which is performed by the digital computer causes the capacity of the arithmetic circuits in the digital computer to be increased.